This invention relates to lasers and more particularly to a method for growing solid state laser crystals boules that are comprised of distinctly different regions with respect to composition. This composition provides single crystal boules that simulate a xe2x80x9csandwich-linexe2x80x9d composite that when made into laser rods do not need separate processing to form end caps onto the laser rods.
There is a high interest in developing efficient lasers in the mid-infrared region for use in various applications including remote sensing, chemical detection and military laser countermeasure systems. In order to increase laser efficiency in this region, efforts have centered on pumping the laser crystals with high power diode lasers that have significantly higher efficiencies than other optical pumps. One approach to high power diode laser pumping is a configuration where the laser crystal is pumped from the ends rather than the sides, as has been common practice in the past. However, end pumping of lasers creates other problems that are discussed in the following paragraphs.
A temperature rise in lasers is a common problem, and particularly with solid state lasers. Such heating is the result of only a percentage of the applied pump energy being converted into useful laser radiation while at least a portion of the remaining energy is transferred into radiationless transitions and absorbed by the host lattice of the laser. To counteract the thermal effects generated by the pump radiation, the laser must be cooled. Typically, laser rods are cooled by direct contact to the outer surface of the rod barrel.
One of the problems encountered with end pumping of lasers is that at high power levels the central region of a laser rod heats to such a degree that thermal expansion causes a significant lens to form at the laser rod faces in response to the large temperature gradient between the hot core and cooled outer surface of the laser rod. This lensing can cause problems with the quality of the output laser beam. More importantly, if the input power is sufficiently high, the large temperature gradient can cause stresses to build up in the laser crystal to the point where fracture of the crystal occurs. This problem is exaggerated at the rod faces, the region where a large fraction of the energy is first absorbed in the laser crystal.
In the past, the problem of thermal lensing and thermal expansion induced stress causing crystal fracture has been solved by diffusion bonding end caps onto a laser crystal, such as taught in U.S. Pat. No. 5,936,984 issued Aug. 10, 1999 to Meissner et al. The end caps contain no dopant, so there is no absorption of diode radiation into the end caps, and there is no thermal lensing due to absorption in the end caps. The thermal lensing still occurs in the region of the doped laser crystal, but the end caps act to conduct the heat of the absorbed energy over a larger volume, thereby redistributing thermal induced stresses and eliminating the fracture problem. The laser rod end caps effectively reduce the thermal gradient and therefore the stress at the outer boundary of the laser rod section. The laser crystal can be operated at equivalent powers without the onset of thermal lensing, or can be operated at much higher average temperatures prior to the formation of significant thermal stresses that result in thermal lensing and/or fracture of the laser crystal.
As currently practiced, diffusion bonding of end caps onto a laser rod adds a significant number of steps to the process cycle. For example, the end caps and the laser rod must be optically ground and polished to a high degree of flatness for the bonding to be effective, making the process for bonding difficult, time consuming, and expensive. In order for a good bond to occur, the optical axes of the laser rod and the end caps must be precisely aligned. Misalignment of the crystal and the end caps can result in changing the polarization vector of the laser beam as it passes through the crystal, resulting in inefficiencies. The surfaces to be bonded must be cleaned to avoid any entrapment of particles. Sometimes, if the bonding is improperly done, the end caps begin to separate from the laser rod. This can result in stress induced fracture failures of the laser crystal.
One technique for bonding separately made end caps onto laser rods is taught in U.S. Pat. No. 5,441,803.
Thus, there is a need in the prior art for a way to eliminate end face fracture of laser rods under high power end pumping of the lasers when the pump energy is focused onto the ends of the laser rods. Although the prior art has demonstrated an approach to fabricate laser rods with end caps, it has been shown to be a very complicated process consisting of many individual steps. Additionally, there is a risk of misalignment of laser rod and end cap components that leads to failure of the laser rod. Accordingly, there is a need to provide composite laser rods using a less complex process.
The stated problems in the prior art are solved by the present invention. A method is disclosed for growing a high power solid state laser crystal that does not experience thermal expansion lensing that can cause crystal fracture under high power pumping on the end of the laser rod without the need for separate end caps as taught in the prior art. To accomplish this, the crystal is grown as a single integral unit with three segments. The crystal fabrication is done at the crystal boule level rather than at the individual laser-rod level. Two segments, the end segments, are un-doped, and they flank a central segment of the crystal that is doped with an active laser ion. The integral end segments replace the separate end caps. This results is significant cost savings because the complexity of preparing separate end caps is eliminated, and preparation and diffusion bonding of the separate end caps onto the laser rod is eliminated. In addition, there is never any misalignment of a crystal and its end caps with resultant potential separation of the end caps from the laser rod.